The present invention relates to digital frequency synthesis, and more particularly to apparatus for correcting the phase of a digitally-synthesized waveform.
Digital waveform synthesis is commonly performed by a numerically controlled oscillator (NCO) comprising an adder and a hold register coupled as an accumulator of fixed size, by incrementing the accumulator with a fixed generator number K at each clock pulse. The number K, denoted the phase increment or frequency vector, is selected such that the accumulator overflows with a period corresponding to the desired synthesized frequency. Except for a synthesized frequency which is a power-of-two submultiple of the clock frequency, however, accumulator overflow will occur at a clock pulse which is offset in time from the precise time of zero crossing, or the leading edge, of the ideal waveform.
Thus, such a numerically controlled oscillator produces a ramp waveform, and the least value and the greatest value of the ramp are each offset from 0 or the accumulator size 2.sup.N by a periodically varying number which is less than the phase increment K. More precisely, for phase increments not equal to 2.sup.n, where n&lt;N, the NCO behaves like a fractional divider with a sequence of non-uniform periods that repeat in a periodic pattern and whose average is the desired synthesized period.
This periodic phase modulation creates undesirable sidebands. For NCOs of the above-described type, one approach in the prior art to eliminating these sidebands has been to employ a high number of amplitude quantization states for the accumulator ramp waveform, and to map the values of the ramp function to a set of corresponding sinusoidal waveform values, by a technique such as using a look-up table. The digital sinusoidal values are then D/A converted, and passed to an analog smoothing element to develop a smoothly varying output waveform. Conventional detection circuitry then interpolates or detects the zero crossings of the converted waveform. This approach, however, is limited by the D/A converter bandwidth.
Another approach to eliminating the sidebands in an NCO-generated waveform has been to employ a high number of amplitude quantization states and to vary the clock phase by addition of a zero mean random clock phase dither. See, for example, U.S. Pat. No. 4,410,954 of C. E. Wheatley, III. This approach is effective only for synthesized frequencies which are several orders of magnitude less than the system clock.
It is desirable to provide a digital frequency synthesizer which provides greater signal fidelity over a broader bandwidth by reducing the power of the sideband frequencies inherently associated with digital waveform synthesis.